This project is concerned with the mechanisms underlying photoreceptor cell responses in the vertebrate retina. Both electrophysiological and biochemical approaches will be employed. During intracellular recordings from photoreceptors of the carp, toad, and when possible, the mudpuppy, the retina will be treated with calcium, cyclic nucleotides, and drugs which specifically affect their metabolism. Assays of cyclic nucleotide concentration will be determined following treatment with drugs which affect their metabolism and following dark and light adaptation. The specific objectives are: 1. to determine the role of calcium in photoreceptor responses and adaptation; 2. to determine the role of cyclic nucleotides in photoreceptor responses and adaptation; 3. to determine the possible interactions between calcium and cyclic nucleotides in photoreceptor responses and adaptation.